Semiconductor modules are well known, in which a plurality of power semiconductor die are fixed to a ceramic based substrate support such as an insulation metal substrate (IMS) or the like to interconnect the devices and are carried in a main support shell which also supports a printed circuit board (PCB) which carries control circuits for controlling the power die. Power terminals extend from the IMS for connection to a load, such as a motor and the PCB carries a terminal connector for connection to an external source of control signals. Such devices, as shown in aforementioned application Ser. No. 09/197,078 are usually arranged so that the IMS is secured within a small opening in the shell (so that the area of the expensive IMS can be minimized) and the bottom surface of the IMS can be pressed into contact with the top flat surface of a heatsink.
The PCB is generally supported in a plane above the plane of the IMS and is laterally removed from the IMS area. The bottom of the PCB is spaced above the top surface of the support shell so that components can be mounted on the bottom surface of the PCB as well as on its top surface.
In general ceramic based substrates are frequently employed to carry the various semiconductor die. These substrates usually have the construction shown in FIGS. 13 and 14 for substrate 320 and have a bottom copper layer 321, a central insulation ceramic 322, which may be Al.sub.2 O.sub.3 or AlN, and a top copper layer which has been patterned into various areas, such as the six insulated areas 323, 324, 325, 326, 327 and 328 shown. Any other pattern could be formed for the top copper layer. Each of areas 323 to 328 have a respective power semiconductor device die 330 to 335 secured thereto, as by soldering or conductive epoxy, or the like. The bottom electrodes of die 330 to 335 are insulated, but could be connected together as desired by conductive traces or by wire bonds. Substrate 320 of FIGS. 13 and 14 may also be a direct bonded copper (DBC) substrate.
In order to insure the mechanical integrity of such substrates, and to prevent the ceramic from cracking, their length is usually limited to less than about 2 inches. Thus, when a power module requires a larger substrate, two or more shorter separate substrates must be used. Thus, as shown in FIG. 15, two identical substrates 320 and 340 are attached to a common base plate 341 which is of copper or of AlSiC for higher performance applications.
Substrates 320 and 340 are conventionally attached to a common baseplate 341 by solder reflow techniques, or by a conductive epoxy. The subassembly of substrates 320, 340 and base plate 341 is then secured within a plastic support shell 350 with the base plate 341 bottom exposed for connection to a flat heatsink 351. A suitable printed current board and terminals are then provided, for example as described in copending application Ser. No. 09/197,078 (IR-1520). The silicon die and substrate are wire bonded or otherwise connected to the PCB and terminal and the substrates are enclosed in a suitable potted volume.
The above described structure has a number of drawbacks. These include:
1. Tooling and material cost for base plate 341.
2. The additional processing required for the use of base plate 341.
3. The added thermal resistance between the silicon die and the heatsink 351 due to the added interfaces at the top and bottom of plate 341.
4. Degradation of power and temperature cycling capability due to the added interfaces.
It would be desirable to employ multiple substrates in a power module without the disadvantages brought about by the added common base plate.
In the known prior art structure and as described above, the entire module is attached to a single unitary heatsink as by screws or the like. The individual devices are electrically isolated from one another against conduction through the common heatsink by the use of the expensive IMS or DBC. The use of the IMS or DBC or the like substrate increases the thermal resistance between the die and heatsink.
It would be desirable to provide a power module which is a self contained circuit, as for a motor control circuit, and which does not require expensive single or multiple insulation substrate(s) and which does not impede heat flow from the die to the heatsink.